1. Field of the Invention
This invention relates generally to glow plugs and, more particularly, to sheathed glow plugs.
2. Related Art
Sheathed heater probe type glow plugs, as shown generally at 1 in prior art FIG. 1, typically have an annular metal shell with a bore 2 sized for fixed receipt of a heater probe assembly 3 therein. The heater probe assembly 3 includes a metallic sheath 4, electrode 5, resistance heating element 6, powder packing material 7, and an o-ring seal 8.
These o-ring seals have been made using various elastomers, including fluoropolymers such as those sold by DuPont under the VITON® brand. While o-ring seals have been used in many glow plug applications, their useful operating temperature range is about 100-200° C., which has now become a constraint in light of engine advancements which have elevated operating temperatures. As a consequence, glow plug applications have been emerging where a higher operating temperature range is needed and the prior art o-ring seals are not suitable.
In addition, even when operating within the lower temperature ranges of the prior art, the o-ring seals 8 are not hermetic and as a result oxygen and water vapor can permeate into the insulating powder 7 and onto the resistance heating element 6 to cause oxidation, cracking and eventually failure of the resistance heating element 6. This failure mode can serve to reduce or otherwise limit the service life of the glow plug 1.
Thus, conventional metallic glow plugs have a limited service life because of degradation of performance due to oxidation of their wire heating element. During thermal cycling, the surface of the wire 6 oxidizes, thereby reducing the effective cross-section of the embedded resistive wire. This eventually leads to overheating of portions of the wire. A major factor leading to this mode of failure is the imperfect seal of the rubber or plastic gasket, e.g., o-ring, which allows oxygen and water vapor to diffuse into the packed powder bed, wherein the oxygen and water vapor are free to react with the heating element wire. Interaction of the magnesium oxide and water vapor form magnesium hydroxide, which can result in failure of the part even before the part is placed into service. Furthermore, gases that are absorbed onto the surface of the magnesium powder may also contribute to the degradation of the heating element wire.
In view of the above, there exists a need for a glow plug that can be used at operating temperatures in the region of the seal above 200° C., and that can provide a true hermetic seal between the electrode and the sheath.